1. Field of the Invention
The present invention relates to an optical element comprising a single crystal which has a light-transmitting end surface, and a process for manufacturing such an optical element. The present invention also relates to a solid-state laser device comprising an optical element as a solid-state laser medium.
2. Description of the Related Art
Conventionally, solid-state laser devices using a single crystal as a solid-state laser medium are known. In such solid-state laser devices, the single crystal is made of Rxe2x80x2:RVO4 or RVO4, where Rxe2x80x2 represents one or a combination of Nd, Er, Tm, Eu, Pr, Ho, Ce, Yb, Dy, Tb or Cr, and R represents Y or Gd. The solid-state laser mediums used in the conventional solid-state laser devices are produced by cutting out a single crystal having a flat light-transmitting end surface, and polishing the cut-out light-transmitting end surface into a mirror finished surface.
Conventionally, the above single crystal is cut out with a plane perpendicular to an a-axis or a c-axis (in other words, parallel with an a-axis or a c-axis) for use as a solid-state laser medium. For example, in the solid-state laser devices of the so-called c-axis end surface excitation type, a single crystal is cut out with a plane perpendicular to an a-axis, and the cut-out surface is polished so that the single crystal is used as a solid-state laser medium, where the solid-state laser devices are configured so that the direction of the linear polarization of the excitation light coincides with the direction of the c-axis of the solid-state laser medium.
However, the light-transmitting end surfaces of the conventional optical elements are prone to damage. Therefore, the yield rates of the conventional solid-state laser devices using the conventional optical elements as a solid-state laser medium are low, and the costs are high.
An object of the present invention is to provide an optical element which includes a single crystal having a light-transmitting end surface, which can be manufactured at a high yield rate and a low cost.
Another object of the present invention is to provide a method for manufacturing an optical element which includes a single crystal having a light-transmitting end surface, whereby the optical element can be manufactured at a high yield rate and a low cost.
A further object of the present invention is to provide a solid-state laser device which comprises an optical element including a single crystal with a light-transmitting end surface, and can be manufactured at a high yield rate and a low cost.
According to the first aspect of the present invention, there is provided an optical element which comprises a single crystal having a flat light-transmitting end surface. In the optical element, the light-transmitting end surface is inclined at at least 0.5 degrees relative to a plane perpendicular to one of an a-axis and a c-axis of the single crystal.
According to the second aspect of the present invention, there is provided a solid-state laser device having an optical element as a solid-state laser medium, wherein the optical element comprises a single crystal having a flat light-transmitting end surface, and the light-transmitting end surface is inclined at at least 0.5 degrees relative to a plane perpendicular to one of an a-axis and a c-axis of the single crystal.
According to the third aspect of the present invention, there is provided a process for producing an optical element which includes a single crystal having a light-transmitting end surface, comprising the steps of: (a) cutting out the single crystal so that the single crystal has a surface which is inclined at at least 0.5 degrees relative to a plane perpendicular to one of an a-axis and a c-axis of the single crystal; and (b) polishing the surface into the light-transmitting end surface.
The present inventor found the cause of the aforementioned problem that the light-transmitting end surface of the conventional optical element is prone to damage. That is, edges of the single crystals of the conventional optical elements are prone to break when the single crystals are cut out with the planes perpendicular to the a-axes and the c-axes, since single crystals are prone to cleave through such planes. Therefore, the edges of the single crystals of the conventional optical elements are prone to break during an operation of polishing cut-out surfaces of the single crystals, and it is probable that the light-transmitting end surfaces are rubbed with fragments of the single crystals. Thus, the light-transmitting end surface of the conventional optical element is prone to damage.
According to the present invention, the light-transmitting end surface is inclined at at least 0.5 degrees relative to the plane perpendicular to one of the a-axis and the c-axis of the single crystal. That is, the light-transmitting end surface is different from the cleavage planes. Therefore, the light-transmitting end surface of the optical element according to the present invention is not prone to break, and it is less probable that the light-transmitting end surfaces are rubbed with fragments of the single crystals during the polishing operation. Thus, according to the present invention, it is possible to prevent the damage of the light-transmitting end surface by the fragments of the crystals produced during the polishing operation, and achieve a high yield rate in production of the optical element.
Since the above high yield rate enables reduction in the price of the optical element, the solid-state laser device using such an optical element as a solid-state laser medium can be manufactured at a low cost.
The effect of increasing the yield rate becomes manifest when the light-transmitting end surface is inclined at at least 0.5 degrees relative to the plane perpendicular to one of the a-axis and the c-axis of the single crystal. The yield rate increases with increase in the inclination angle above 0.5 degrees, and the rate of increase in the yield rate becomes moderate when the inclination angle exceeds 5 degrees. When the inclination angle exceeds 20 degrees, the yield rate reaches its ceiling.
However, when the above optical element is used as a solid-state laser medium in some solid-state laser devices, it is not preferable to increase the inclination angle of the light-transmitting end surface in respect of luminous efficiency and the like. Since the yield rate reaches its ceiling when the inclination angle exceeds 20 degrees, it is preferable that the inclination angle of the light-transmitting end surface relative to the plane perpendicular to the one of the a-axis and the c-axis of the single crystal is at most 20 degrees. It is further preferable that the inclination angle of the light-transmitting end surface relative to the plane perpendicular to the one of the a-axis and the c-axis of the single crystal is at most 5 degrees.
In addition, in each aspect of the present invention, one or any combination of the following additional features may be provided.
(1) The single crystal may have a tetragonal lattice structure.
(2) The single crystal may be made of Rxe2x80x2:RVO4 or RVO4, where Rxe2x80x2 represents one or a combination of Nd, Er, Tm, Eu, Pr, Ho, Ce, Yb, Dy, Tb or Cr, and R represents Y or Gd.
(3) The light-transmitting end surface is inclined so that the light-transmitting end surface and a plane perpendicular to an a-axis may make an angle of at least 0.5 degrees around the c-axis.
(4) The light-transmitting end surface is inclined so that the light-transmitting end surface and a plane perpendicular to an a-axis may make an angle of at least 0.5 degrees around another a-axis of the single crystal.
Further, in the solid-state laser device according to the second aspect of the present invention, the following additional features may be provided.
(5) When the above additional feature (3) is provided, the solid-state laser device may be configured so that the direction of linear polarization of excitation light which excites the solid-state laser medium is parallel with the c-axis of the single crystal.
(6) When the above additional feature (4) is provided, the solid-state laser device may be configured so that the direction of linear polarization of the excitation light which excites the solid-state laser medium is perpendicular to the said another a-axis, which is contained in the light-transmitting end surface.
When the light-transmitting end surface and a plane perpendicular to the a-axis make an angle of at least 0.5 degrees around the c-axis, and the direction of linear polarization of the excitation light which excites the solid-state laser medium is parallel with the c-axis of the single crystal, the advantages of the great cross-section of stimulated emission and the great absorption coefficient are obtained, as in the case of the usual solid-state lasers of the c-axis end surface excitation type, and are not affected by the increase in the inclination angle of the light-transmitting end surface relative to the plane perpendicular to the a-axis of the single crystal. That is, the luminous efficiency and the like are not reduced by the increase in the inclination angle of the light-transmitting end surface relative to the plane perpendicular to the a-axis of the single crystal, as long as the direction of linear polarization of the excitation light which excites the solid-state laser medium is parallel with the c-axis of the single crystal.
When the light-transmitting end surface and a plane perpendicular to the a-axis make an angle of at least 0.5 degrees around the other a-axis of the single crystal, and the direction of linear polarization of the excitation light which excites the solid-state laser medium is perpendicular to the other a-axis, which is contained in the light-transmitting end surface, the direction of linear polarization of the excitation light includes a c-axis component (component in the direction of the c-axis) and an a-axis component (component in the direction of an a-axis). In order to exploit, as much as possible, the advantages of the great cross-section of stimulated emission and the great absorption coefficient, it is necessary to increase the c-axis component of the direction of linear polarization of the excitation light. That is, when the light-transmitting end surface and a plane perpendicular to the a-axis make an angle of at least 0.5 degrees around another a-axis of the single crystal, and the direction of linear polarization of the excitation light which excites the solid-state laser medium is perpendicular to the other a-axis, which is contained in the light-transmitting end surface, it is preferable that the inclination angle of the light-transmitting end surface relative to the plane perpendicular to the a-axis of the single crystal is as small as possible.